Equine Infectious Anemia: Why do we test and why should you care?

By Garrett Metcalf, DVM

What is a Coggins test, and why is it so important? Why do I need a Coggins test if I am moving my horse(s) out of state or to an event? Why is the test only good for one year? This article will discuss the history of Equine Infectious Anemia and why it is a very important disease to keep under control. It will also discuss the route of transmission from one horse to another and the symptoms that a horse will have if it gets the disease.

Equine Infectious Anemia has been known by many names: Swamp Fever, Slow Fever, Mountain Fever, Equine Malarial Fever, and Coggins Disease, or EIA. It is a lentivirus from the family Retroviridae, and can infect all members of the Equidae family from ponies, donkeys, mules, and horses. EIA has been recognized as a disease of horses for centuries, but it made its big emergence in the 1930s and reached its height of devastation between the 1960s and 1970s. More than 10,000 horses were identified to be infected in 1975, and many of those horses died from the disease. Today the disease is less prevalent, and horses that do have the disease are nearly all-asymptomatic carriers of the disease, showing no signs of the disease. These horses act as reservoirs of the disease to spread to others.

The most common transmitting vector for EIA is biting flies, most commonly horse flies or deer flies. These flies lacerate the skin of horses to suck up a blood meal. The blood of an infected horse will remain on the mouth parts of these flies for a short period of time but long enough to be transmitted to an uninfected horse. Other ways of transmission is sharing hypodermic needles between horses, and it has even been found to be transmitted from mare to foal in utero.  Other minor routes of transmission can be from semen, milk and possibly be aerosolization of infectious material.

The clinical signs or symptoms of EIA are often nonspecific and usually the only sign is a fever. In severe cases horses will become weak, depressed, have increase heart rate and respiration rate, ventral edema, anemia, and bleeding from nostrils, and blood in their stools. Some cases will die during the acute phase of the disease, but those that recover will become asymptomatic persistently infected carriers. EIA is difficult to differentiate from other fever-producing diseases such as anthrax, influenza and equine encephalitis.

There is no treatment and no vaccines for EIA. The viral genome of EIA rapidly mutates, making it very difficult to create an effective vaccine to EIA.  Once a horse contracts EIA, the horse will always have the disease.

The only method to stop the spread of the disease is by prevention, and the only options to manage infected horses are to quarantine the infected horses at least 200 yards from healthy horses or to euthanize them. That is why testing is key to controlling the spread of this disease. Coggins testing is required once or twice a year depending on the state you live in, before traveling out of state, before entering an organized event, or sale of horses. It is always recommended to get a negative Coggins test before you introduce a new horse into a herd setting to maintain an EIA-free herd.

The Coggins test, developed in 1973 by Dr. Leroy Coggins, a graduate of Oklahoma State College of Veterinary Medicine in 1957, helps detect infected horses with EIA using the AGID method, or Agar Gel Immunodiffusion Assay. Dr. Coggins developed this test while studying viruses at Cornell University.  Today, the AGID test has been replaced mostly by the ELISA method. which is the most common test, used in reference labs around the country. The original method of testing of AGID is still considered the “gold standard” internationally.

If a horse is moved internationally they are required to get a Coggins test with the AGID method. A negative Coggins test is required before a health certificate is issued for travel. Generally most states require a negative Coggins test within one year, but some require it every six months. Veterinarians accredited by the USDA are the only veterinarians allowed to do Coggins testing and issue health certificates. Health certificates are issued to insure the horses that are traveling to events or crossing state lines are healthy and allow a level of traceability if a horse does become sick.  Most health certificates are issued for 30 days, but some are only issued for 10 days. A Coggins test requires hand drawn or digital images of your horse, identifying markings, address the horse resides, breed, age, sex, and owner’s information to complete the test. Today with a service called GlobalVetLINK hand drawn images are replaced with digital images uploaded into the Coggins form.

Luckily, with lots of hard work on behalf of veterinarians, laboratories, and state officials and due diligent horse owners, EIA is rather under control today, but there are still new cases of EIA discovered routinely. Also, there are many horses that do not receive testing, leading to possible reservoir of horses asymptomatically carrying EIA. Remember, even though it is sometimes inconvenient to get your horse’s Coggins test performed, you are doing your part to help control and prevent the spread of this terrible and incurable disease.

Read more great stories in the April 2020 issue of Oklahoma Farm & Ranch.